Many types of orthopedic devices include hinges that support joints, and control and limit joint movements. These joints include the knee, elbow, shoulder, hip, ankle and wrist joints.
The knee joint, although frequently considered a hinge joint, comprises two joints, lateral and medial, between the femur and tibia, and one arthrodial joint between the patella and femur. The primary movements of the knee comprise flexion, i.e., rearward rotational movement of the tibia relative to the femur, and extension, when the knee is straight and the angle between the femur bone and the tibia bone is 0 degrees.
The flexion and extension movements of the knee joint are not simply pivotal movements about a fixed axis. During flexion, the axis around which movement takes place shifts backward, and during extension it shifts forward. This differs from a more typical hinge joint, such as an elbow, where the axis of rotation does not shift. As full extension is reached, the tibia is rotated inward or rearward and the joint in effect is disposed in a “locked” position with the ligaments taut. This gives the joint greater stability in the extended position. As flexion is initiated, the tibia initially lowers or moves downwardly with small external rotation of the tibia unlocking the joint and subsequently the tibia rotates or rolls about the joint to full flexion. The initial unlocking of the knee joint during flexion precedes actual full rotation of the knee.
Because of the complexity associated with knee movement, a knee brace hinge mechanism should be able to simulate the movements of the knee. Incorporating such a hinge mechanism is important, as the knee brace must optimally support the knee joint of its user.
In post-surgical applications, the requirement for such simulation of the knee joint is important to rehabilitate and prevent re-injury of an injured knee joint. In recognizing the need for an effective post-surgical knee brace, various types of hinge mechanisms have been incorporated into known knee braces for supporting and controlling movement of the knee. One type of hinge used for duplicating movement of the human knee joint in an orthopedic device is a plural axis or polycentric hinge. As taught in U.S. Pat. Nos. 4,524,764, 4,732,143, and 5,443,444, a polycentric hinge is used to support the knee joint throughout its full range of motion, or to lock the knee joint in a selected position, or to limit the allowed range of motion of the joint to less than complete extension (straightening) and or less than complete flexion (bending) of the knee.
Because of this complexity repeat movement of the knee, the need to support the knee through a range of motion, and the ability to lock or restrict movement of knee, existing polycentric knee brace hinges are complex, including numerous parts that must be assembled together, making such devices complex and relatively expensive.
It would be an advance within the art to provide a polycentric hinge of increased simplicity, while still providing the beneficial functionality associated with existing polycentric hinge structures.